U.S. patent application number 12/685649 was filed with the patent office on 2011-07-14 for control channel interference management and extended pdcch for heterogeneous network.
This patent application is currently assigned to RESEARCH IN MOTION LIMITED. Invention is credited to Zhijun Cai, Andrew Mark Earnshaw, Mo-Han Fong, Youn Hyoung Heo, Sean McBeath, Robert Novak, Jack Anthony Smith, Hua Xu.
Application Number | 20110170496 12/685649 |
Document ID | / |
Family ID | 44258453 |
Filed Date | 2011-07-14 |
United States Patent
Application |
20110170496 |
Kind Code |
A1 |
Fong; Mo-Han ; et
al. |
July 14, 2011 |
Control Channel Interference Management and Extended PDCCH for
Heterogeneous Network
Abstract
A method for managing control channel interference is provided.
The method includes a first access node performing at least one of
blanking and transmitting at lower than nominal transmit power on
at least a portion of a control channel. The method further
includes the first access node applying at least one of blanking
and transmitting at lower than nominal transmit power only on the
control region of chosen subframes, wherein a second access node
transmits the control region of the chosen subframes at nominal
transmit power.
Inventors: |
Fong; Mo-Han; (Kanata,
CA) ; Heo; Youn Hyoung; (Kitchener, CA) ; Xu;
Hua; (Kanata, CA) ; Smith; Jack Anthony;
(Irving, TX) ; Earnshaw; Andrew Mark; (Kanata,
CA) ; Novak; Robert; (Kanata, CA) ; Cai;
Zhijun; (Irving, TX) ; McBeath; Sean; (Irving,
TX) |
Assignee: |
RESEARCH IN MOTION LIMITED
Waterloo
CA
|
Family ID: |
44258453 |
Appl. No.: |
12/685649 |
Filed: |
January 11, 2010 |
Current U.S.
Class: |
370/329 |
Current CPC
Class: |
H04W 52/244 20130101;
H04W 52/325 20130101; H04W 52/44 20130101; H04W 52/04 20130101;
H04W 72/082 20130101; H04W 52/38 20130101; H04L 5/0053 20130101;
H04W 52/24 20130101; H04W 52/143 20130101 |
Class at
Publication: |
370/329 |
International
Class: |
H04W 72/08 20090101
H04W072/08 |
Claims
1. A method for managing control channel interference comprising: a
first access node performing at least one of blanking and
transmitting at lower than nominal transmit power on at least a
portion of a control channel; and the first access node applying at
least one of blanking and transmitting at lower than nominal
transmit power only on the control region of chosen subframes,
wherein a second access node transmits the control region of the
chosen subframes at nominal transmit power.
2. The method of claim 1, wherein, on the chosen subframes of the
first access node, the second access node schedules at least one
user equipment (UE).
3. The method of claim 1, wherein the first access node transmits
system information on subframes other than the chosen
subframes.
4. The method of claim 1, wherein the chosen subframes are one of:
in a fixed pattern within a certain time duration; and dynamically
changing on the subframe level.
5. The method of claim 1, wherein the first access node is the
access node for a closed subscriber group (CSG) cell, and wherein
the first access node blanks at least a portion of the control
channel on the chosen subframes on at least one of the component
carriers.
6. The method of claim 5, wherein the first access node transmits
scheduling assignments to at least one UE on an extended PDCCH
(E-PDCCH).
7. The method of claim 5, wherein the first access node transmits
cross carrier scheduling assignments to at least one UE using the
control channel on component carriers whose control channel is not
blanked.
8. A method for managing control channel interference comprising: a
second access node providing a special subframe configuration to a
first access node via at least one of: X2 signaling, S1 signaling,
a self organizing network (SON) manager, and a system information
block, wherein the first access node transmits a control channel to
at least one UE during at least one of the special subframes of the
second access node.
9. The method of claim 8, wherein the special subframe is a
Multicast Broadcast Single Frequency Network (MBSFN) subframe.
10. The method of claim 8, wherein the first access node determines
the special subframe configuration of the second access node by
reading a system information block of the second access node to
look for an information element describing the special subframe
configuration.
11. The method of claim 8, wherein the first access node directs
its transmissions to at least one UE that is more severely
interfered with by the second access node during the special
subframes.
12. The method of claim 8, wherein the second access node blanks
transmission of at least one of a data channel and a multicast data
channel on at least one OFDM symbol that corresponds to the control
region of the first access node on at least one of the special
subframes.
13. The method of claim 9, wherein the second access node transmits
multicast data channel service data only on certain special
subframes, and wherein the second access node does not transmit the
control channel in those special subframes, and wherein the second
access node informs the first access node of the special subframes
via at least one of X2 signaling, S1 signaling, and a SON manager,
and wherein the first access node directs at least some of its
transmissions into the special subframes.
14. The method of claim 13, wherein the first access node directs
its transmissions to at least one UE that is more severely
interfered with by the second access node during the special
subframes.
15. A first access node, comprising: a processor configured such
that the first access node performs at least one of blanking and
transmitting at lower than nominal transmit power on at least a
portion of a control channel, and applies at least one of blanking
and transmitting at lower than nominal transmit power only on the
control region of chosen subframes, wherein a second access node
transmits the control region of the chosen subframes at nominal
transmit power.
16. The first access node of claim 15, wherein, on the chosen
subframes of the first access node, the second access node
schedules at least one user equipment (UE).
17. The first access node of claim 15, wherein the chosen subframes
of the first access node are not used to transmit system
information by the first access node.
18. The first access node of claim 15, wherein the chosen subframes
are one of: in a fixed pattern within a certain time duration; and
dynamically changing on the subframe level.
19. The first access node of claim 15, wherein the first access
node is the access node for a closed subscriber group (CSG) cell,
and wherein the first access node blanks at least a portion of the
control channel on the chosen subframes on at least one of the
component carriers.
20. The first access node of claim 19, wherein the first access
node transmits scheduling assignments to at least one UE on an
extended PDCCH (E-PDCCH).
21. The first access node of claim 19, wherein the first access
node transmits cross carrier scheduling assignments to at least one
UE using the control channel on component carriers whose control
channel is not blanked.
22. A second access node, comprising: a processor configured such
that the second access node provides a special subframe
configuration to a first access node via at least one of: X2
signaling, S1 signaling, a self organizing network (SON) manager,
and a system information block, wherein the first access node
transmits a control channel to at least one UE during at least one
of the special subframes of the second access node.
23. The second access node of claim 22, wherein the special
subframe is a Multicast Broadcast Single Frequency Network (MBSFN)
subframe.
24. The second access node of claim 22, wherein the first access
node determines the special subframe configuration of the second
access node by reading a system information block of the second
access node to look for an information element describing the
special subframe configuration.
25. The second access node of claim 22, wherein the first access
node directs its transmissions to at least one UE that is more
severely interfered with by the second access node during the
special subframes.
26. The second access node of claim 22, wherein the second access
node blanks transmission of at least one of a data channel and a
multicast data channel on at least one OFDM symbol that corresponds
to the control region of the first access node on at least one of
the special subframes.
27. The second access node of claim 23, wherein the second access
node transmits multicast data channel service data only on certain
special subframes, and wherein the second access node does not
transmit the control channel in those special subframes, and
wherein the second access node informs the first access node of the
special subframes via at least one of X2 signaling, S1 signaling,
and a SON manager, and wherein the first access node tries to
direct at least some of its transmissions into the special
subframes.
28. The second access node of claim 27, wherein the first access
node directs its transmissions to at least one UE that is more
severely interfered with by the second access node during the
special subframes.
Description
BACKGROUND
[0001] As used herein, the terms "user equipment" and "UE" can
refer to wireless devices such as mobile telephones, personal
digital assistants, handheld or laptop computers, and similar
devices that have telecommunications capabilities. Such a UE might
consist of a wireless device and its associated Universal
Integrated Circuit Card (UICC) that includes a Subscriber Identity
Module (SIM) application, a Universal Subscriber Identity Module
(USIM) application, or a Removable User Identity Module (R-UIM)
application or might consist of the device itself without such a
card. The term "UE" may also refer to devices that have similar
wireless capabilities but that are not transportable, such as
desktop computers, set-top boxes, or network appliances. The term
"UE" can also refer to any hardware or software component that can
terminate a communication session for a user. Also, the terms "user
equipment," "UE," "user agent," "UA," "user device" and "user node"
might be used synonymously herein.
[0002] As telecommunications technology has evolved, more advanced
network access equipment has been introduced that can provide
services that were not possible previously. This network access
equipment might include systems and devices that are improvements
of the equivalent equipment in a traditional wireless
telecommunications system. Such advanced or next generation
equipment may be included in evolving wireless communications
standards, such as Long Term Evolution (LTE) and LTE-Advanced
(LTE-A). For example, an LTE or LTE-A system might include an
Evolved Universal Terrestrial Radio Access Network (E-UTRAN) node B
(eNB), a wireless access point, or a similar component rather than
a traditional base station. As used herein, the terms "eNB" or
"access node" will refer to any component of a wireless network,
such as a traditional base station, a wireless access point, or an
LTE or LTE-A eNB, that creates a geographical area of reception and
transmission coverage allowing a UE or a relay node to access other
components in a telecommunications system. An access node may
comprise a plurality of hardware and software. The term "legacy"
may be used herein to refer to hardware, software, and standards
related to LTE versions prior to LTE-A Release 10.
[0003] A traditional high power access node can create a relatively
large area of coverage that can be referred to as a macro cell.
Several different types of low power access nodes have been
introduced that have a smaller area of coverage than a macro cell
and that might improve the quality and reliability of the coverage
provided by a macro cell. These low power access nodes include
remote radio heads, pico eNBs, home eNBs (HeNBs), relay nodes, and
similar components. These terms might be used interchangeably
herein, or any such access node might be referred to generically as
a low power node. An access node that creates a macro cell might be
referred to herein as a macro eNB. The area of coverage created by
a low power node might be referred to as a micro cell, a pico cell,
a femto cell, a hotzone cell, a relay cell, a low power cell, or a
similar term, any of which might be used interchangeably herein.
Also, when a reference is made herein to an action being taken by a
macro cell or by a low power cell, it should be understood that the
action might be taken by the access node associated with that
cell.
[0004] One difference between low power nodes and macro eNBs is a
different propagation environment. In particular, low power nodes
might be deployed indoors and might have different antenna heights
and antenna patterns compared with macro eNBs. Also, the transmit
power might be different for macro eNBs and low power nodes. For
example, the maximum allowable transmit powers of a macro eNB, a
remote radio head/pico, a HeNB, an outdoor relay, and an indoor
relay are typically 46 dBm, 30 dBm, 20 dBm, 30 to 37 dBm, and 20
dBm, respectively, for a 10 MHz carrier bandwidth.
[0005] Heterogeneous networks are one of the important features in
LTE-A. Heterogeneous networks consist of deployments where low
power nodes are placed throughout a macro cell layout. As described
below, a UE in a heterogeneous network might obtain higher quality,
more reliable, or less costly service by attaching to a low power
node rather than a macro eNB.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] For a more complete understanding of this disclosure,
reference is now made to the following brief description, taken in
connection with the accompanying drawings and detailed description,
wherein like reference numerals represent like parts.
[0007] FIG. 1 is a diagram showing a PDCCH (Physical Downlink
Control Channel) region and a PDSCH (Physical Downlink Shared
Channel) region in portions of subframes, according to the prior
art.
[0008] FIG. 2 illustrates blanking/muting of a control region for a
heterogeneous deployment, according to the prior art.
[0009] FIGS. 3a and 3b illustrate reduced transmit power on a macro
cell for a heterogeneous deployment, according to the prior
art.
[0010] FIG. 4 illustrates a shift of a HeNB subframe relative to
that of a macro cell, according to the prior art.
[0011] FIG. 5 illustrates extending a PDCCH transmission region in
a backward compatible component carrier, according to the prior
art.
[0012] FIGS. 6a and 6b illustrate reduced transmit power on a macro
cell control region for a heterogeneous deployment, according to an
embodiment of the disclosure.
[0013] FIG. 7 illustrates the relative transmit power levels of the
control channels, the CRS (Common Reference Signal), and the PDSCH,
according to an embodiment of the disclosure.
[0014] FIG. 8 illustrates the relative transmit power levels of the
control channels, the CRS, the PDSCH for a legacy UE, and the PDSCH
for an LTE-A UE, according to an embodiment of the disclosure.
[0015] FIG. 9 illustrates an E-PDCCH (Extended PDCCH) transmission
by a low power node, according to an embodiment of the
disclosure.
[0016] FIG. 10 illustrates a reference signal design for an
E-PDCCH, according to an embodiment of the disclosure.
[0017] FIG. 11 illustrates an embodiment of a method for managing
control channel interference, according to an embodiment of the
disclosure.
[0018] FIG. 12 illustrates an embodiment of a method for managing
control channel interference, according to an alternative
embodiment of the disclosure.
[0019] FIG. 13 illustrates an embodiment of a method for managing
control channel interference, according to an alternative
embodiment of the disclosure.
[0020] FIG. 14 illustrates an embodiment of a method for managing
control channel interference, according to an alternative
embodiment of the disclosure.
[0021] FIG. 15 illustrates a processor and related components
suitable for implementing the several embodiments of the present
disclosure.
DETAILED DESCRIPTION
[0022] It should be understood at the outset that although
illustrative implementations of one or more embodiments of the
present disclosure are provided below, the disclosed systems and/or
methods may be implemented using any number of techniques, whether
currently known or in existence. The disclosure should in no way be
limited to the illustrative implementations, drawings, and
techniques illustrated below, including the exemplary designs and
implementations illustrated and described herein, but may be
modified within the scope of the appended claims along with their
full scope of equivalents.
[0023] For both LTE Release 8 (Rel-8) and LTE-A on a backward
compatible carrier, the control region consists of the physical
control format indicator channel (PCFICH), the physical downlink
control channel (PDCCH), and the physical HARQ (hybrid automatic
repeat request) indicator channel (PHICH). The PCFICH contains the
Control Format Indicator (CFI) which carries information about the
number of orthogonal frequency division multiplexing (OFDM) symbols
(1, 2 or 3 for bandwidths greater than 1.4 MHz) used for the
transmission of PDCCHs in the corresponding subframe. The PCFICH is
transmitted on four resource element (RE) groups (REGs) where each
REG consists of four contiguous REs. A resource element is the most
basic unit of transmission that can carry a modulation symbol and
is essentially one subcarrier in the frequency domain for the
duration of one OFDM symbol. These four REGs are spread over the
whole system bandwidth in the first OFDM symbol of corresponding
subframe.
[0024] The PDCCH is transmitted by the serving eNB to its UE to
convey physical downlink shared channel (PDSCH) and physical uplink
shared channel (PUSCH) scheduling grant information. The downlink
PDSCH scheduling grant can be designated to a particular UE for
dedicated PDSCH resource allocation to carry UE-specific traffic,
or it can be designated to all UEs in the cell for common PDSCH
resource allocation to carry broadcast control information such as
system information or paging. In a normal subframe, for system
bandwidth other than 1.5 MHz, up to three OFDM symbols can be used
as the PDCCH region (as shown in FIG. 1). In a multicast/broadcast
single frequency network (MBSFN) subframe, up to two OFDM symbols
can be used as the PDCCH region. Each PDCCH consists of one, two,
four or eight Control Channel Elements (CCE) where each CCE
consists of nine REGs. The REGs are interleaved across the system
bandwidth as well as across the OFDM symbols defined for the PDCCH
region. A cyclic shift is applied to the interleaved REGs based on
the cell ID of each cell.
[0025] The PHICH carries downlink HARQ acknowledgement/negative
acknowledgement (ACK/NACK) information in response to an uplink
PUSCH transmission. The PHICH occupies REGs within the PDCCH
region. The number of OFDM symbols occupied by the PHICH is defined
by the PHICH duration signaled in the Master Information Block
(MIB) where the MIB is carried in the Physical Broadcast Channel
(PBCH). The PHICH duration can be less than the number of OFDM
symbols defined for the PDCCH region indicated by the CFI. The REGs
occupied by the PHICH are also spread across the system bandwidth
and the number of OFDM symbols corresponding to the PHICH
duration.
[0026] As described above, REs that form a control channel (PCFICH,
PDCCH, PHICH) are spread across the system bandwidth, and for the
cases of the PDCCH and the PHICH, those REs are also spread across
the OFDM symbols defined for the corresponding control channels.
Therefore, the frequency division multiplexing type of inter-cell
interference avoidance, i.e., reserving different REs or resource
blocks (RBs) for control channel transmission in different cells,
is not possible. A resource block represents a set of REs and has
dimensions of 12 contiguous subcarriers in the frequency dimension
by 1 slot in the time dimension. There are two slots in each 1 ms
subframe.
[0027] There are several issues or challenges that are associated
with heterogeneous deployments. As described above, low power nodes
have a significantly lower transmit power than macro eNBs, i.e.,
9-16 dB lower power for the case of outdoor low power nodes and 26
dB lower power for the case of indoor low power nodes. In the
downlink, a UE served by a low power cell will be severely
interfered with by the overlaid macro cell. Interference management
or avoidance can be performed on the PDSCH region since different
RBs can be reserved and/or the power can be controlled for use by
the macro cell and the low power cell to their own UEs. However,
similar interference management or avoidance cannot be performed
for the control channels due to the REs that are used to transmit a
control channel (i.e., PCFICH, PDCCH, PHICH) being spread across
the system bandwidth as described above. In addition, robust
reception of the PDCCH (and thus the PCFICH) is more crucial than
for the PDSCH for three reasons. First, a UE needs to receive the
PDCCH correctly in order to know the PDSCH resource assigned and
subsequently perform PDSCH decoding. Second, a UE needs to decode
the PCFICH correctly so that it can decode the PDCCH correctly and
also know the PDCCH region boundary and therefore know the starting
symbol of PDSCH. Third, as opposed to the PDSCH, HARQ
retransmission cannot be applied to the PDCCH.
[0028] Another challenge associated with heterogeneous deployments
is that in a typical homogeneous network, a UE is attached (or
served) by the strongest cell, i.e., the cell with the strongest
downlink received signal strength. Due to the significantly lower
transmit power of the low power nodes, low power cells will have a
very small service area if the same rule is applied to decide
whether a UE should attach to a macro cell or a low power cell.
[0029] There are various reasons for a UE to attach to a low power
cell even though the low power cell does not provide the strongest
downlink received signal strength. First, when a UE has a lower
coupling loss (including large scale path loss, shadowing, antenna
gain, etc.) to a low power cell compared to a macro cell, it may be
better for the UE to attach to the low power cell, although the UE
receives a stronger downlink signal from the macro cell due to the
significantly larger transmit power of the macro cell. This
minimizes the required uplink transmit power from the UE and the
resultant uplink interference the UE causes to other UEs in the
neighbor cells.
[0030] Another reason for a UE to attach to a low power cell is
that, with such low transmit power, low power cells typically have
very small coverage areas. Therefore, low power nodes distributed
in various locations within a macro cell coverage area represent a
cell-splitting type of frequency reuse among non-collocated low
power cells. Overall, a resource occupied by a UE that is served by
a low power cell is much less costly to the macro cell than would
be the same resource occupied by a UE that is served by a macro
cell. Therefore, it may be desirable to increase the service area
and the number of UEs served by a low power cell.
[0031] Another reason for a UE to attach to a low power cell is
that a low power node may be a HeNB cell (also called a femto
cell). A UE that is subscribed to a Closed Subscriber Group (CSG)
to which the HeNB belongs may get special or cheaper services when
it is served by the HeNB. When a UE is attached to a low power cell
for the above reasons, the interference experienced by the UE from
the overlaid macro cell might be very high.
[0032] In another scenario, a UE attached to a macro cell can
experience severe interference from the low power cell. This can
occur when the lower power cell is a femto cell in closed access
mode where the UE is not a subscriber of the corresponding CSG. In
this case, even though the downlink received signal strength from
the femto cell is the strongest, the UE is not allowed to attach to
the femto cell.
[0033] A number of solutions have been proposed to address the
PDCCH interference issues in heterogeneous networks. One proposed
solution is a carrier aggregation based solution, where an
extension carrier is defined with blanking/muting of the PDCCH
region. This solution is shown in FIG. 2. Interference on femto
cell control region transmissions (PDCCH, PCFICH, PHICH) can be
mitigated by configuring extension carriers with cross-carrier
scheduling. An extension carrier is a carrier that does not
transmit the control region. As shown in FIG. 2, for UEs attached
to the macro cell (e.g. UE1), available bandwidth can be split into
two component carriers (CCs), CC1 and CC2, with CC1 configured as
the primary carrier containing all the critical control signaling
and with CC2 configured as an extension carrier. CC1 can be
accessible to both legacy (LTE Release 8) and advanced (LTE Release
10) UEs. Legacy LTE UEs will be unaware of the existence of the
extension carrier. Advanced LTE-A UEs can be configured to receive
data on the extension carrier (in addition to the primary carrier)
using cross-carrier PDSCH assignments received in the control
region of the primary carrier. This allows the macro cell to "free
up" the control region of CC2 from interfering with the low power
cell due to the high power of the downlink PDCCH/PHICH/PCFICH
transmissions. Since the CC2 control region is free from
interference caused by macro eNB transmissions, various femto cells
within the coverage area of the macro cell can configure their
control region to overlap (in the time domain and the frequency
domain) with the control region of the macro cell and configure CC2
as their primary carrier. If the bandwidth corresponding to CC1 is
also available for the femto cells, they can further configure CC1
as an extension carrier and schedule CC1 PDSCH resources using
cross-carrier scheduling assignments from CC2.
[0034] For the PDSCH region on CC1 and CC2, interference from the
macro cell to the femto cell can be mitigated by RB-based
interference management/avoidance techniques. Alternatively, the
macro cell and the femto cell can each lower their PDSCH transmit
power on their corresponding extension carrier, i.e., CC2 and CC1
respectively. In this way, the interference from the macro cell on
the Primary/Secondary Synchronization Signal (PSS/SSS) and PBCH
transmissions of femto cells can be reduced and vice versa.
[0035] This solution has two main drawbacks. First, although this
solution significantly reduces the interference from the macro cell
to the control region of the low power cell, a new type of carrier
(i.e., extension carrier) has to be defined. Since the control
region is `blanked` or not transmitted in the extension carrier,
this extension carrier cannot be used to support legacy Rel-8 UEs.
Second, this solution increases the loading of the PDCCH on the
primary (or non-extension carrier). Since the PDCCH region is
limited to a maximum of three OFDM symbols, the PDCCH resources may
not be sufficient to support all of the PDSCH assignments on
multiple CCs.
[0036] Another proposed solution to address the PDCCH interference
issues in heterogeneous networks is also a carrier aggregation
based solution. In this case, the transmit power of certain
carriers is reduced. That is, instead of blanking of the
transmission of the control region on an extension carrier
(PDCCH-less carrier), it has been proposed that the macro cell
reduce the transmit power on the extension carrier. This is shown
in FIG. 3.
[0037] In the macro cell, a higher transmit power is used on CC1
while a lower transmit power is used on CC2. In the femto/relay
cell, the transmit power is low on both CCs. Since PDCCHs on CC1 in
the femto/relay cell suffer strong interference from the macro
cell, a PDCCH-less operation is configured for CC1 in the
femto/relay cell. Although the PDSCH resources may suffer a similar
strong interference on CC1 in the femto/relay cell as the PDCCH,
the PDSCH resources can be efficiently utilized by an RB-level
interference coordination between the macro cell and femto/relay
cell depending on the traffic load of the macro cell and the
femto/relay cell. It has also been proposed that it may be useful
to configure a PDCCH-less operation for CC2 in the macro cell
because transmitting PDCCHs from CC1 is more efficient and
reliable.
[0038] There are two main drawbacks to this solution. First,
reducing the transmit power of one of the carriers on the macro
cell does not help if the pico/femto cell deployment is not at the
cell edge. In particular for a femto cell, the deployment can
potentially be anywhere within the macro cell coverage area.
Second, in the case where a pico/femto cell is deployed at the cell
edge, this solution can work, but there is no reason to reduce the
transmit power of the macro eNB on CC2 for the PDSCH region as
well. This unnecessarily reduces the coverage and capacity of the
PDSCH.
[0039] Yet another proposed solution to address the PDCCH
interference issues in heterogeneous networks is also a carrier
aggregation based solution. In this case, UEs in different
locations are served by different carriers. More specifically, it
has been proposed that the CC1 discussed above does not need to be
a PDCCH-less carrier in the pico/femto cell. It can be used to
serve UEs that are close to the pico/femto eNB and experience low
interference from the macro cell.
[0040] This solution has drawbacks similar to those of the first
and second solutions described above when blanking/muting of the
PDCCH is performed on CC2 by the macro cell or when the macro cell
transmit power on CC2 is reduced, respectively.
[0041] Yet another proposed solution to address the PDCCH
interference issues in heterogeneous networks is a non-carrier
aggregation based solution. In this case, the control region is
time shifted between the macro cell and the low power cell. That
is, instead of using carrier aggregation and different treatment
(e.g., blanking or power management) on different CCs, it has been
proposed that the subframe timing between the macro cell and the
HeNB cell be shifted by k symbols. This is shown in FIG. 4 and FIG.
5.
[0042] As shown in FIG. 4 (where the vertical axis represents
frequency and the horizontal axis represents time), the proposed
solution uses time shifting of the HeNB transmission by k symbols
(i.e., to avoid overlap with the macro eNB control region, n1) and
uses macro eNB power reduction or muting on the portion of a symbol
(or symbols) that overlap the control region of the HeNB. The macro
eNB could also use power reduction on all the RBs (i.e., the 25 RBs
for the case of 5 MHz system bandwidth) overlapping the HeNB
control region to improve PDSCH performance for HeNBs very close to
the macro eNB. It has been proposed that a single OFDM symbol HeNB
control region (n2=1) is preferred for PDSCH efficiency. This
leaves five CCEs for HeNB control channels, which should be
sufficient for HeNB control signaling. Due to the time shift of the
HeNB transmissions, the last n1 symbols of the HeNB PDSCH region
would see interference from the macro eNB control region. The HeNB
PDSCH overlapped with the macro cell control region could be
further mitigated by either (a) using truncation so that only
14-n2-n1 symbols would be used for the HeNB PDSCH or (b) not using
truncation (i.e., using 14-n2 symbols for HeNB PDSCH) but
accounting for the overlap via the selection of an appropriate
modulation and coding scheme.
[0043] It has also been proposed that an additional one subframe
shift (k=14+n1=16 total symbols) is needed so that the HeNB's
PSS/SSS/PBCH do not overlap with the macro eNB's PSS/SSS/PBCH. Then
the macro cell would have to mute or attenuate its PDSCH symbol(s)
overlapping the HeNB control region and would also attenuate or
mute RBs that overlap the HeNB's PSS/SSS/PBCH.
[0044] This solution has three main drawbacks. First, muting or
reduced power on a few symbols of the PDSCH region of the macro eNB
will severely degrade the PDSCH performance, in particular for a
Rel-8 UE that is unaware of the muting or reduced power. Second,
truncation of the last few symbols in the PDSCH region of the HeNB
will severely degrade the performance for a Rel-8 UE. Third, this
solution is dependent on full timing synchronization between the
HeNB and the macro eNB.
[0045] Another proposed solution to address the PDCCH interference
issues in heterogeneous networks is also a non-carrier aggregation
based solution. In this case, it has been proposed to define a new
extended PDCCH (E-PDCCH) region in the PDSCH region. In general,
several key objectives have been proposed for the E-PDCCH. First,
the resources available for the PDCCH are extended, in particular
for carrier aggregation as well as for other features that also
increase the required resources for the PDCCH, such as multiple
user multiple input/multiple output (MU-MIMO). Second, interference
mitigation of the PDCCH for heterogeneous networks might be
performed as the E-PDCCH is defined in the PDSCH region, and
therefore RB-based interference management and coordination can be
performed.
[0046] The proposals related to this solution do not provide a
great deal of detail, but most of them indicate reuse of the
existing PDCCH structure and the relay PDCCH (R-PDCCH) design for
the backhaul link between the donor eNB and the relay node. A
general structure of the E-PDCCH is shown in FIG. 5. The E-PDCCH
region can be time division multiplexed (TDM) and/or frequency
division multiplexed (FDM) with the existing PDSCH region, where a
number of RBs (contiguous or non-contiguous) and a number of OFDM
symbols within those RBs are used for the E-PDCCH.
[0047] Primary and secondary PDCCHs have been proposed, where the
primary PDCCH is located in the legacy PDCCH region and consists of
new downlink control information (DCI) that points to the secondary
(i.e., extended) PDCCH region. The new DCI is scrambled by an LTE-A
radio network temporary identifier (RNTI), i.e., an RNTI defined
for all LTE-A UEs.
[0048] As mentioned above, a drawback of this solution is that not
much detail has been provided on how the E-PDCCH would be used in a
heterogeneous network. Also, no consideration has been given on the
impact of TDM/FDM between the E-PDCCH and PDSCH regions on the
reference signal design.
[0049] In an embodiment, six solutions are provided that address
the PDCCH interference issues in heterogeneous networks and the
drawbacks of the existing solutions for those issues.
[0050] The first solution is a carrier aggregation based solution
wherein a reduced transmit power is used on the control region only
of the macro cell. Three concepts under this solution address the
PDCCH interference problems described above and some of the
drawbacks of the existing carrier aggregation based solutions to
those problems. In the first concept under the first solution, the
macro eNB transmit power is reduced on the control region but the
transmit power on the PDSCH region is not reduced accordingly, so
the performance of the PDSCH is not impacted while the interference
to the control region of low power nodes is reduced. In the second
concept under the first solution, methods are provided to reduce
the control region transmit power without reducing the transmit
power on the Common Reference Signal (CRS). This supports a
non-reduced PDSCH coverage and data rate for Rel-8 UEs. In the
third concept under the first solution, methods are provided to
reduce the control region transmit power and corresponding CRS
transmit power. This provides uniform coverage of the PDCCH and
PDSCH for Rel-8 UEs. Details of each of these concepts will now be
provided.
[0051] In an embodiment of the first concept under the first
solution, the transmit power is reduced on the control region
(i.e., PCFICH, PDCCH, PHICH) only on one or a subset of the CCs of
the macro cell. The PDSCH transmit power of those CCs is not
reduced accordingly; however, RB-based power management can be
performed for inter-cell interference coordination with the low
power nodes. This is illustrated in FIGS. 6a and 6b. On CC2 610,
the macro eNB 620 reduces the transmit power on its control region
such that the interference introduced to the control region of the
low power node 630 is small. To effectively reduce the control
region interference, the number of OFDM symbols in the control
region of the low power node 630 is set equal to or less than that
number for the macro eNB 620. This is practically achievable since
typically the number of UEs served by a low power node is much
lower than the number of UEs served by a macro eNB, and therefore
the low power nodes generally require smaller PDCCH regions.
[0052] To achieve this, in one embodiment, the macro eNB transmits
at the maximum possible PDCCH region size (for example, the macro
eNB uses three or four OFDM symbols). In this way, there will be no
or much reduced interference from the PDCCH region of the macro eNB
to the PDCCH of the low power node. In another embodiment, the low
power node transmits the PDCCH in a number of OFDM symbols that is
less than or equal to N. N is configured by the network through
signaling (e.g., the macro eNB signals to the low power node via S1
or X2 interface or a self organizing network (SON) manager signals
to the low power node) and hence the macro eNB will transmit the
PDCCH region with a number of OFDM symbols that is equal to or
larger than N. In another embodiment, N may be pre-configured. The
second alternative may be more attractive since it is configurable
(based on the loading, the interference situations, etc.) and more
spectrum efficient. To enable the alternative, the low power node
may need to report the interference measurement, average loading,
etc., to the network (e.g., to the macro eNB via S1/X2 signaling or
to the SON manager) to select a suitable N.
[0053] Due to the reduced control region transmit power on CC2 610,
the control channel coverage 640 of the macro cell on CC2 610 is
much reduced as shown in FIG. 6a. For a Rel-8 or LTE-A UE (e.g.,
UE1 650 in FIG. 6a) served by the macro cell which is located close
to the macro eNB 620 and within the control channel coverage 640 of
CC2 610, the macro eNB 620 can transmit the PDCCH on CC2 610 to the
UE 650 to schedule a PDSCH assignment on CC2 610. That is, there is
no need for cross carrier scheduling assignment. For a LTE-A UE
(e.g., UE2 660 in FIG. 6a) served by the macro cell which is
located outside of the control channel coverage of CC2 610, the
macro eNB 620 can transmit the PDCCH on CC1 670 to the UE 660 to
schedule PDSCH assignment on CC2 610. That is, cross carrier
scheduling assignment is used.
[0054] In this way, even though the macro cell control channel
coverage on CC2 is reduced, the macro cell PDSCH coverage, data
rate, and capacity for LTE-A UEs are not affected. In addition,
since the macro cell control region on CC2 is not blanked/muted,
CC2 can be used to serve legacy Rel-8 UEs, albeit with reduced
coverage. It should be noted that Rel-8 does not support the cross
carrier scheduling assignment feature. Therefore, a Rel-8 UE
located outside of the CC2 control channel coverage is assigned
PDSCH resources on CC1 only through a PDCCH scheduling assignment
transmitted on CC1.
[0055] In the second concept under the first solution, the control
region transmit power is reduced without reducing the transmit
power on the CRS. CRS is a reference signal used for channel
estimation and PDCCH/PDSCH demodulation by Rel-8 UEs. The CRS is
also used by LTE-A UEs for channel estimation and demodulation of
the PDCCH. A UE-specific demodulation RS (DM-RS) is used in LTE-A
for channel estimation and demodulation of the PDSCH.
[0056] In this concept, in an embodiment, even though the control
channel transmit power is reduced as in the first concept of the
first solution, (e.g., on CC2 of the macro eNB), the CRS is
transmitted at non-reduced nominal power. For a Rel-8 UE, the PDSCH
is transmitted at a power level that is offset from the CRS
transmit power, where the offset value is configured per UE and
ranges from -6 dB to 3 dB. Since the transmit power of the CRS is
not reduced, the transmit power of the PDSCH for Rel-8 UEs does not
need to be reduced, and therefore the coverage is not impacted.
[0057] This concept is illustrated in FIG. 7. The control channels
710 are transmitted at a reduced power relative to the CRS 720,
which is transmitted at a non-reduced nominal transmit power,
P.sub.0. The PDSCH 730 is transmitted at a UE-specific transmit
power that is equal to P.sub.0 plus an offset.
[0058] There are several issues related to maintaining a high
transmit power on the CRS while reducing the control channel
transmit power. One issue is that, for the RRC_Connected mode,
Rel-8 UEs and possibly LTE-A UEs might use the CRS to measure the
reference signal received power/reference signal received quality
(RSRP/RSRQ) to trigger measurement reports to the eNB. The UEs
might be outside of the coverage of the control channels on CC2
while the measured RSRP/RSRQ from the CRS is still at an acceptable
level. As a result, a measurement report may not be triggered, and
the eNB may not know that the UE has moved out of the control
channel coverage of CC2.
[0059] A second issue is that, for the RRC_Idle mode, Rel-8 UEs and
possibly LTE-A UEs might use the CRS for measuring RSRP/RSRQ to
perform cell ranking for cell reselection purposes. If the existing
Rel-8 cell ranking criteria (as shown below) are used, the UE may
reselect a cell (carrier) to camp on even when the UE is outside of
the PDCCH coverage of the carrier. The cell-ranking criterion
R.sub.s for a serving cell s and the cell-ranking criterion R.sub.n
for a neighboring cell n are defined by:
R.sub.s=Q.sub.meas,s+Q.sub.hyst
R.sub.n=Q.sub.meas,n-Qoffset
where Q.sub.meas is the RSRP measurement quantity used in cell
reselections, and Q.sub.hyst specifies the hysteresis value for
ranking criteria. For intra-frequency, Qoffset equals
Qoffset.sub.s,n if Qoffset.sub.s,n is valid; otherwise, Qoffset
equals zero. For inter-frequency, Qoffset equals Qoffset.sub.s,n
plus Qoffset.sub.frequency if Qoffset.sub.s,n is valid; otherwise,
Qoffset equals Qoffset.sub.frequency. Qoffset.sub.s,n specifies the
offset between the two cells, i.e. the serving cell s and the
neighboring cell n. Qoffset.sub.frequency is a frequency-specific
offset for equal priority E-UTRAN frequencies.
[0060] For the cell selection case in the RRC_Idle mode, a Rel-8 UE
and possibly a LTE-A UE follow the Rel-8 criterion for selecting
the cell to camp on based on the measured RSRP value. If the UE is
outside of the PDCCH coverage of a cell (carrier) but the cell
selection criterion has been met due to the higher CRS transmit
power, the UE will not be able to subsequently locate and decode
the System Information Blocks (SIBs) and therefore will not select
the cell (carrier) to camp on.
[0061] A third issue is that the high power transmission of the CRS
by the macro eNB might interfere with the CRS transmitted by the
low power nodes. In Rel-8, there are only three possible location
shifts for the CRS in each subframe for collision avoidance of the
CRS among neighbor cells if at least two CRS antenna ports are
allocated for a cell. With a potential high density deployment of
low power nodes, there is a high likelihood that the CRSs between
the macro eNB and the low power nodes will collide. If a collision
occurs, the SINR of the CRS transmitted by the low power node will
be severely degraded, and thus the channel estimation performance
of UEs attached to the low power node will be severely
impacted.
[0062] In an embodiment, four possible remedies to address the
first issue described above are provided for Rel-8 and LTE-A UEs.
In a first remedy, the eNB configures one or more scaled RSRP/RSRQ
thresholds for the UE to apply to the measurement results on CC2,
in order to trigger measurement and measurement reports on CC2. The
level of scaling corresponds to the power reduction on the control
channels. In one embodiment, the eNB configures a scaled-up
s-Measure value, where s-Measure, as defined in the Third
Generation Partnership (3GPP) Technical Specification (TS) 36.331,
is the serving cell (carrier) signal quality threshold controlling
whether or not the UE is required to perform measurements of
intra-frequency, inter-frequency, and inter-RAT (Radio Access
Technology) neighboring cells. The UE is also configured with an
intra-frequency or inter-frequency measurement object and reporting
configuration corresponding to the carrier frequency of other
available component carriers, such as CC1 in the example provided
with respect to the first concept under the first solution. In an
embodiment, the inter-frequency measurement reporting criteria for
CC1 can be based on Event A5 defined in 3GPP TS 36.331. That is,
measurements are reported when the serving cell (carrier)
measurement result becomes worse than threshold1 and the neighbor
cell (carrier) measurement result becomes better than threshold2.
In this case, the eNB configures a scaled-up threshold1 value.
Based on the measurement report received from the UE, the eNB can
decide whether to hand over the UE from CC2 to CC1. The other
inter-frequency measurement reporting criteria, such as events A4,
A3, etc., can also be used.
[0063] In a second remedy, instead of relying on the UE's
intra-frequency or inter-frequency measurements and measurement
reports, the eNB can compare the UE's CQI (Channel Quality
Indicator) report on CC2 with an appropriate threshold that takes
into consideration the reduced control channel power. The eNB can
use the comparison to decide whether to hand over the UE to another
CC with a larger coverage, such as CC1 in the example provided with
respect to the first concept under the first solution. If CC1 and
CC2 are in the same frequency band, the CQI report from the UE on
CC2 provides a sufficiently close estimate of the long-term signal
quality on CC1. If CC1 and CC2 are in different frequency bands,
the eNB can apply a predefined correction factor to the CQI
reported from the UE on CC2 to estimate the long-term signal
quality on CC1.
[0064] In addition, for LTE-A UEs that support carrier aggregation,
further possible remedies to address the first issue can be
applied. In a third remedy, an LTE-A UE is configured (or assigned)
multiple CCs, such as CC1 and CC2, as shown in the example provided
with respect to the first concept under the first solution. When
the UE is in the control channel coverage of CC2, only CC2 is
activated. That is, the UE only performs signal reception on CC2
during the active time on CC2.
[0065] In one embodiment under this third remedy, the UE is
configured to perform intra-frequency measurement on CC1 and CC2.
The intra-frequency measurement reporting criteria configured on
CC2 are such that a measurement report is triggered when the
RSRP/RSRQ on CC2 is below a certain threshold, e.g., Event A2 as
defined in 3GPP TS 36.331. The intra-frequency measurement
reporting criteria configured on CC1 are such that a measurement
report is triggered when the RSRP/RSRQ on CC1 is above a certain
threshold, e.g., Event A1 as defined in 3GPP TS 36.331. Based on
these measurement reports, the eNB can decide when to activate CC1
and deactivate CC2 for the UE.
[0066] In another embodiment under this third remedy, the UE is
configured to perform intra-frequency measurement on CC2, which is
the serving cell (or carrier). The UE is configured with a
measurement object that corresponds to the carrier frequency of CC1
to perform inter-frequency measurement on the non-serving cell (or
carrier), i.e., CC1 in this example. The inter-frequency
measurement reporting criteria configured for CC1 are such that a
measurement report is triggered when the serving cell (i.e., CC2)
RSRP/RSRQ is below a certain threshold and the inter-frequency cell
(i.e., CC1) RSRP/RSRQ is above a certain threshold, e.g., Event A5
as defined in 3GPP TS 36.331. Based on these measurement reports,
the eNB can decide when to activate CC1 and deactivate CC2 for the
UE.
[0067] In another embodiment under this third remedy, no intra- or
inter-frequency measurement object is configured for CC2. The eNB
can compare the UE's CQI report on CC2 with an appropriate
threshold that takes into consideration the reduced control channel
power in deciding whether to hand over the UE to another CC with a
larger coverage, e.g., CC1 in the example provided with respect to
the first concept under the first solution. If CC1 and CC2 are in
the same frequency band, the CQI report from the UE on CC2 provides
a sufficiently close estimate of the long-term signal quality on
CC1. If CC1 and CC2 are in different frequency bands, the eNB can
apply a predefined correction factor to the CQI reported from the
UE on CC2 to estimate the long-term signal quality on CC1.
[0068] In a fourth remedy, an LTE-A UE is configured (or assigned)
multiple CCs, e.g., CC1 and CC2 as shown in the example provided
with respect to the first concept under the first solution. When
the UE is in the control channel coverage of CC2, both CC1 and CC2
are activated. That is, the UE performs signal reception on CC1 and
CC2 during the active time of CC1 and CC2, respectively.
[0069] In one embodiment under this fourth remedy, the UE is
configured to perform intra-frequency measurement on CC1 and CC2.
The intra-frequency measurement reporting criteria configured on
CC2 are such that a measurement report is triggered when the
RSRP/RSRQ on CC2 is below a certain threshold, e.g., Event A2 as
defined in 3GPP TS 36.331. Based on the measurement report, the eNB
can decide when to deactivate CC2 for the UE.
[0070] In another embodiment under this fourth remedy, the UE is
configured with a measurement object to perform intra-frequency
measurement on CC2. The UE is configured with a measurement object
that corresponds to the carrier frequency of CC1. The measurement
reporting criteria configured for the measurement object of CC1 are
such that a measurement report is triggered when a reference
serving cell (e.g. CC2) RSRP/RSRQ is below a certain threshold and
a neighbor cell (e.g. CC1) RSRP/RSRQ is above a certain threshold,
e.g., similar to Event A5 as defined in 3GPP TS 36.331. Based on
these measurement reports, the eNB can decide when to deactivate
CC2 for the UE.
[0071] In another embodiment under this fourth remedy, no
intra-frequency measurement object is configured for CC2. The eNB
can compare the UE's CQI report on CC2 with an appropriate
threshold that takes into consideration the reduced control channel
power in deciding whether to deactivate CC2. If CC1 and CC2 are in
the same frequency band, the CQI report from the UE on CC2 provides
a sufficiently close estimate of the long-term signal quality on
CC1. If CC1 and CC2 are in different frequency bands, the eNB can
apply a predefined correction factor to the CQI reported from the
UE on CC2 to estimate the long-term signal quality on CC1.
Alternatively, the eNB configures the UE to report CQI on both CC1
and CC2.
[0072] In an embodiment, two possible remedies are provided to
address the second issue described above regarding the RRC_Idle
mode. A first remedy to the second issue can be applied to Rel-8
UEs as well as LTE-A UEs. The eNB broadcasts (e.g., in SIB4) an
adjusted value of Qoffset.sub.s,n that accounts for the reduced
control channel transmit power with respect to the CRS transmit
power. For example, when CC2 is the serving cell s camped on by the
UE, the Qoffset.sub.s,n value broadcast by CC2 is a negative value
that accounts for the transmit power delta between the CRS and the
control channel. This negative value means the Q.sub.meas,n of the
neighbor cell n, which does not have a transmit power imbalance
between the CRS and the control channel, is effectively boosted by
|Qoffset.sub.s,n|. When CC2 is a neighbor cell n being searched by
the UE, i.e., the UE is camped on another serving cell s (e.g.,
CC1) that does not have a transmit power imbalance between the CRS
and the control channel, the Qoffset.sub.s,n value broadcast by CC1
that corresponds to the neighbor cell n on CC2 is a positive value
that accounts for the transmit power delta between the CRS and the
control channel in CC2. This positive value means the neighbor
cell's Q.sub.meas,n is effectively de-boosted by |Qoffset.sub.s,n|.
The adjusted value of Qoffset.sub.s,n may need to be updated when
the power reduction in the serving cell and the neighboring cells
is changed. When the serving cell s and neighbor cell n both
correspond to carriers that have the same transmit power imbalance
between the CRS and the control channel, the Qoffset.sub.s,n value
broadcast by the serving cell does not need to account for the
power delta between the CRS and the control channel. When the
serving cell s and neighbor cell n both correspond to carriers that
have a different transmit power imbalance between the CRS and the
control channel, the Qoffset.sub.s,n value broadcast by the serving
cell needs to account for the power delta between the CRS and the
control channel.
[0073] A second remedy to the second issue applies to LTE-A UEs
only, as the Rel-8 cell reselection is modified to the
following:
R.sub.s=Q.sub.meas,s+Q.sub.hyst-Q.sub.control.sub.--.sub.offset,
s
R.sub.n=Q.sub.meas,n-Qoffset-Q.sub.control.sub.--.sub.offset, n
where Q.sub.meas the RSRP measurement quantity used in cell
reselections. For intra-frequency, Qoffset equals Qoffset.sub.s,n
if Qoffset.sub.s,n is valid; otherwise, Qoffset equals zero. For
inter-frequency, Qoffset equals Qoffset.sub.s,n plus
Qoffset.sub.frequency if Qoffset.sub.s,n is valid; otherwise,
Qoffset equals Qoffset.sub.frequency.
Q.sub.control.sub.--.sub.offset, s is the offset applied to
Q.sub.meas of the serving cell to account for the reduced control
channel transmit power with respect to the CRS transmit power.
Q.sub.control.sub.--.sub.offset, n is the offset applied to
Q.sub.meas of a neighbor cell to account for the reduced control
channel transmit power with respect to the CRS transmit power.
These parameters are signaled to the UE via the broadcast control
channel or possibly by the dedicated channel.
[0074] To resolve the third issue described above, the low power
node can monitor the neighbor cells' interference conditions and
choose the appropriate CRS shift that would result in the least
interference from neighbor cells. However, with potential high
density deployment of low power nodes, CRS interference from the
macro cell and/or neighbor low power cells may still be
inevitable.
[0075] In an alternate remedy to resolve the third issue, a third
concept is provided wherein a reduced control region transmit power
and corresponding CRS transmit power are used by the macro eNB. In
this embodiment, the transmit powers of the control channels and
the CRS are reduced by the same nominal amount. In this way, the
CRS transmitted by the macro eNB does not interfere with the CRS
transmitted by the low power nodes even when the locations of the
CRS REs collide. Furthermore, this approach does not incur problems
associated with the second concept described above for
RRC_Connected mode measurement report and RRC_Idle mode cell
reselection. For a Rel-8 UE, the PDSCH is transmitted at a power
level that is offset from the CRS transmit power, where the offset
value is configured per UE and ranges from -6 dB to 3 dB. Since the
transmit power of the CRS is reduced, the transmit power of the
PDSCH for Rel-8 UEs is also reduced, and therefore the data rate
and coverage of the PDSCH for a Rel-8 UE is also reduced similarly
to that of the control channels. For an LTE-A UE, since a
UE-specific DM-RS is used for PDSCH demodulation, the transmit
power of the PDSCH is not tied to that of the CRS. The PDSCH for an
LTE-A UE can be transmitted at non-reduced power or at a power
suitable for PDSCH interference coordination between the macro cell
and low power cells.
[0076] This is concept illustrated in FIG. 8. The control channels
810 and the CRS 820 are transmitted at the same reduced transmit
power, P.sub.crs. The PDSCH 830 for a Rel-8 UE is transmitted at a
transmit power that is offset from P.sub.crs. The PDSCH 840 for an
LTE-A UE is transmitted at a transmit power that is independent of
the CRS transmit power.
[0077] Rel-8 UEs use the CRS to measure RSRP/RSRQ. Therefore, the
measured RSRP/RSRQ will accurately reflect the control channel
coverage. Unlike in the second concept described above, the eNB
does not need to scale the RSRP/RSRQ thresholds in the measurement
reporting criteria configured for the UE.
[0078] Similarly, an LTE-A UE can also use the CRS to measure
RSRP/RSRQ for measurement reporting and cell selection/reselection
purposes. Alternatively, an LTE-A UE can use the channel state
information reference signal (CSI-RS) to measure RSRP/RSRQ. The
CSI-RS is a low-density RS defined for LTE-A UEs to measure CQI and
channel state information for Multiple Input Multiple Output (MIMO)
channel feedback purposes. In an embodiment, the CSI-RS is
transmitted at non-reduced power since it represents the achievable
PDSCH transmit power for LTE-A UEs. In an embodiment, the
information broadcast by the eNB on an SIB, such SIB Type 2 (SIB2),
includes the CRS power or Energy per Resource Element (EPRE) and
the CSI-RS power or EPRE.
[0079] An LTE-A UE can be configured to use the CRS or the CSI-RS
for RSRP/RSRQ measurement and reporting. In one embodiment, in a
backward compatible CC, the eNB can configure the UE via RRC (Radio
Resource Control) signaling to use either the CRS or the CSI-RS for
measurement and measurement reporting. In a non-backward compatible
CC or extension CC, the UE is configured to use the CSI-RS for
measurement and measurement reporting.
[0080] When an LTE-A UE is configured to perform measurement and
measurement reporting based on the CSI-RS, similar schemes as
provided with regard to the second concept described above can be
applied to account for the reduced control channel transmit power
compared to the CSI-RS transmit power.
[0081] In an embodiment, a second solution that addresses the PDCCH
interference issues in heterogeneous networks and the drawbacks of
the existing solutions for those issues is a non-carrier
aggregation based solution wherein low power nodes transmit an
extended PDCCH (E-PDCCH). Five concepts under this solution address
the PDCCH interference problems described above in a non-carrier
aggregation manner, since spectrum may not be abundant for the
operators. This solution also addresses some of the drawbacks of
the existing non-carrier aggregation based solutions described
above.
[0082] In a first concept under this second solution, a low power
node transmits the E-PDCCH and blanks/mutes or does not transmit
the legacy control channel region. In a second concept under the
second solution, a low power node transmits the E-PDCCH as well as
the legacy control channel region. This allows the low power node
to support legacy Rel-8 UEs. In a third concept under the second
solution, the E-PDCCH region is configured with and without
blanking of the legacy control channel region. In a fourth concept
under the second solution, a new type of non-backward compatible
carrier is defined for heterogeneous networks. In a fifth concept
under the second solution, the macro eNB blanks/mutes certain OFDM
symbols within its legacy control region or the PDSCH region to
reduce the interference to low power nodes.
[0083] In each of these concepts, it may be preferable for
co-channel (i.e., non-carrier aggregation) interference avoidance
schemes to be supported since operators may not have sufficient
spectrum to support multiple CCs, with some CCs being backward
compatible and others not being backward compatible (e.g.,
extension carriers). Details of each of these concepts will now be
provided.
[0084] In the first concept, the low power node transmits the
E-PDCCH and blanks/mutes or does not transmit the legacy control
channel region. The macro eNB transmits the legacy control channel
region with full power, thus providing uniform control channel
coverage for UEs in the macro cell area. To avoid interference from
the macro eNB, in one embodiment, low power nodes located within
the coverage area of the macro eNB do not transmit on the legacy
control channel region. Instead, the E-PDCCH defined within the
PDSCH region is transmitted by the low power node to carry PDSCH
and PUSCH scheduling assignment information to UEs attached to the
low power node. In one embodiment, the E-PDCCH transmission within
the PDSCH region could be restrained in the frequency and time
domains (and even in the RBs) to allow more controllable
interference coordination. In this case, the low power node may not
support legacy Rel-8 or Rel 9 UEs.
[0085] In the second concept, the low power node transmits the
E-PDCCH. The low power node also transmits the control channels in
the legacy control channel region for UEs which are located close
to the low power node and experience low interference from the
macro eNB. This is shown in FIG. 9 as `assignment1` 910. For UEs
which are located farther away from the low power node 920 and
experience higher interference from the macro eNB 930, their
control channel information is carried in the E-PDCCH. This is
shown in FIG. 9 as `assignment2 940`. Since legacy Rel-8 UEs do not
support the E-PDCCH, when a legacy Rel-8 UE is close to the low
power node 920, it is served by the low power node 920. When a
legacy UE is farther away from the low power node 920, it is served
by the macro eNB 930. On the other hand, LTE-A UEs can be served by
the low power node 920 with a larger coverage area due to the use
of the E-PDCCH. In some embodiments, the `assignment 1` 910 not
only contains the scheduling information for the UE, but also
contains the scheduling information of the `assignment 2` 940. In
this case, it may be preferable to use the highest CCE aggregation
level for `assignment 1` 910.
[0086] In the third concept, the E-PDCCH region is configured with
and without blanking of the legacy control channel region. When the
control channel region of a low power node is not blanked, the
PDCCH transmitted in the legacy control channel region can carry a
new DCI defined to carry information related to the configuration
of the E-PDCCH region, such as the resources (e.g., in terms of RBs
and/or the number of OFDM symbols) allocated to the E-PDCCH region,
the CCE aggregation level used in the E-PDCCH region, etc. In one
embodiment, the resources of the E-PDCCH region could simply be
defined by the RBs indicated by the new DCI. For example, one RB
could carry the E-PDCCH information. TDM/FDM multiplexing within a
subframe can be applied to the multiplexing of the PDSCH region and
one or more E-PDCCH regions. In that case, the starting position
and the ending position of the OFDM symbols within those assigned
RBs for an E-PDCCH region may be semi-statically notified through
RRC signaling. In another embodiment for the case of TDM/FDM
multiplexing, the starting position and the ending position of the
OFDM symbols within those assigned RBs for an E-PDCCH region may be
also notified by the new DCI.
[0087] A large CCE aggregation level (greater than 8 CCEs) can be
used for the new DCI to improve the robustness of its reception by
UEs located farther away from the low power node. In one
embodiment, an LTE-A UE served by the low power node can be
semi-statically configured (e.g., through RRC signaling) to blind
decode a set of aggregation levels which may be different from
those defined for Rel-8 (i.e., aggregation levels 1-8). For
example, such an LTE-A UE could be configured to blind decode
aggregation levels 4-16 in the legacy PDCCH region in order to
obtain extra coding gain on the PDCCH broadcast by the low power
node.
[0088] An E-PDCCH group RNTI can be introduced to scramble the new
DCI. (Scrambling of a DCI is defined as scrambling the CRC (Cyclic
Redundancy Check) value corresponding to the DCI contents with a
RNTI value.) The E-PDCCH group RNTI is assigned to a group of LTE-A
UEs that need to decode the E-PDCCH for scheduling assignment
information. There can be multiple E-PDCCH regions defined for
different groups of LTE-A UEs. Each of these E-PDCCH regions is
indicated by a new DCI transmitted in the legacy control region.
Each of the new DCIs is scrambled by the corresponding E-PDCCH
group RNTI. It is also possible that multiple DCIs may be
transmitted over the same region.
[0089] Another option under the third concept is to define a new
E-PCFICH channel with a predefined resource location within the
legacy control region or the PDSCH region to carry the
location/size information of the E-PDCCH region.
[0090] Alternatively, when the control channel region of a low
power node is blanked, or even if it is not blanked, the E-PDCCH
region information can be signaled to the UEs via broadcast,
multi-cast (using the E-PDCCH group RNTI), or dedicated RRC
signaling.
[0091] In another embodiment under the third concept, a combination
of new DCI and RRC signaling can be used to indicate the E-PDCCH
region information. For example, the RRC signaling could indicate
the semi-static configuration, such as the RBs used for the E-PDCCH
region and the transmission format (e.g. MIMO transmission mode,
aggregation levels of the E-PDCCH etc.), whereas the new DCI could
indicate the number of symbols within those RBs that are used for
the E-PDCCH region. In another example, the new DCI could indicate
the start and end index of symbols within those RBs that are used
for the E-PDCCH region. In yet another example, the RRC signaling
could indicate the different possible sets of RBs used for the
E-PDCCH region, whereas the new DCI could indicate the specific set
of RBs used for the E-PDCCH region for the corresponding subframe
where the new DCI is transmitted.
[0092] In one embodiment, the size of this new DCI is set to the
size of one of the existing Rel-8 DCI formats in order to reduce
the decoding complexity. In another embodiment, the new DCI is
confined within a reduced search space of the PDCCH region, e.g.,
the common search space.
[0093] In the fourth concept, a new type of non-backward compatible
carrier is defined. In this new carrier, the legacy control channel
region is blanked. That is, there is no legacy PCFICH, PDCCH, or
PHICH transmission. Also, one or more E-PDCCH regions are
configured. The partial or full configuration of some or all of the
E-PDCCH regions can be signaled through broadcast RRC signaling
such as the MIB. This type of carrier is fully accessible by LTE-A
UEs and can be camped on by LTE-A UEs in Idle mode. It can also be
assigned to LTE-A UEs as a standalone CC. A UE accessing the
carrier/cell would first read the MIB to understand the
configuration of one or more E-PDCCH regions and then subsequently
read SIB1, SIB2, etc., based on the PDSCH scheduling assignment
sent on those E-PDCCH regions. One or more of the E-PDCCH regions
could also carry the LTE-A PHICH for sending a downlink ACK/NACK to
acknowledge an uplink PUSCH transmission from a UE.
[0094] In the fifth concept, the macro eNB blanks/mutes certain
OFDM symbols within its legacy control region or the PDSCH region.
Both the macro eNB and the HeNB transmit in the legacy control
region. To aid in the service of Rel-8 UEs by the HeNB, the macro
eNB could signal (through the PCFICH) that in a subframe, it will
have N symbols for PDCCH transmission, but could apply additional
coding to the PDCCHs (i.e., increase the aggregation level) and
puncture the entire Nth symbol. To avoid the impact of blanking on
the PHICH, the PHICH duration can be limited to being less than N
symbols, which can be configured by RRC signaling. In the subframe
for which that occurs, the HeNBs could also indicate (through the
PCFICH) N (or more) symbols for the legacy control region. Because
of the puncturing of the Nth symbol by the macro eNB, the REs in
the Nth symbol at the HeNBs would experience improved SINR, thus
improving a Rel-8 UE's reception of the HeNB PDCCHs. The decision
by the macro eNB to puncture the Nth symbol of the control region
could occur in response to a request from the HeNB or could occur
according to a pre-arranged pattern of subframes. In one
embodiment, the subframe start position of the HeNB is time shifted
so as to align with what would normally be expected to be the
punctured Nth symbol of the macro eNB control region so that when
the macro eNB punctures the Nth symbol, the first symbol of the
HeNB control region would experience no interference.
[0095] In another embodiment under the fifth concept, the macro eNB
could advertise (through the PCFICH) that in a subframe, it will
have N symbols for PDCCH transmission. The HeNB could advertise
that in the same subframe, it will have M symbols for PDCCH
transmission, where M is greater than N. The macro eNB could blank
the PDSCH transmission on the (N+1)th to Mth symbols within its
subframe. This will reduce the interference caused to the HeNB's
control region from symbol (N+1) to symbol M. To reduce the impact
of PDSCH blanking on Rel-8 UEs, a more conservative modulation and
coding set can be assigned to the Rel-8 UEs because the performance
of PDSCH will be degraded due to the puncturing of PDSCH coded
bits. For LTE-A UEs, the eNB can send separate signaling (e.g., RRC
signaling or a new DCI) to inform those UEs of the symbols on which
PDSCH blanking occurs or the actual starting OFDM symbol of the
PDSCH transmission. In this case, the LTE-A UEs receive PDSCH
transmission on the symbols which are not blanked and can avoid the
puncturing loss.
[0096] In an embodiment, a third solution that addresses the PDCCH
interference issues in heterogeneous networks and the drawbacks of
the existing solutions for those issues is, like the second
solution, a non-carrier aggregation based solution. In the second
solution, the use of the E-PDCCH for control channel interference
management was described. In the third solution, additional
concepts for the E-PDCCH are provided.
[0097] Five concepts under this solution address the PDCCH
interference problems described above in a non-carrier aggregation
manner, since spectrum may not be abundant for the operators. This
solution also addresses some of the drawbacks of the existing
non-carrier aggregation based solutions described above.
[0098] The first concept deals with the design of the DM-RS for the
E-PDCCH to support adequate channel estimation performance of the
E-PDCCH. The second concept deals with the uplink HARQ ACK/NACK
timing for the PDSCH assigned by the E-PDCCH. The third concept
provides methods to support different transmission modes for the
E-PDCCH which can benefit different UEs' location and channel
conditions. The fourth concept uses a cross carrier indication for
the E-PDCCH region. The fifth concept deals with the transmission
of the E-PDCCH on the blanked/muted OFDM symbols by a macro eNB
within its legacy control region or the PDSCH region. Details of
these five concepts will now be provided.
[0099] As previously described, an E-PDCCH region may consist of
multiple contiguous or non-contiguous RBs and may occupy a subset
of OFDM symbols within those RBs. An E-PDCCH may consist of CCEs
and REGs that spread across the allocated RBs and OFDM symbols
within the E-PDCCH region, in a similar fashion as the interleaving
procedure of the Rel-8 PDCCH. To provide good channel estimation
for each of the REGs/CCEs that form an E-PDCCH, in an embodiment of
the first concept under the third solution, an RB-based DM-RS
design is defined for the E-PDCCH. This means that each RB within
the E-PDCCH region consists of sufficient DM-RS for good channel
estimation within the RB.
[0100] In an embodiment, a DM-RS pattern similar to those defined
for the LTE-A PDSCH can be reused for the E-PDCCH DM-RS. The
E-PDCCH region and the PDSCH region share the same set of available
DM-RS RE locations within an RB for their respective DM-RS
transmissions. An example of the E-PDCCH region and an E-PDCCH
DM-RS pattern within a normal subframe (i.e., non-MBSFN subframe)
and in a backward compatible carrier is shown in FIG. 10. Two of
the RBs 1010 reserved for the E-PDCCH region are shown in the
figure. The first four of the OFDM symbols 1020 within these two
RBs 1010 are reserved for the E-PDCCH. REGs 1030 that form a CCE of
the E-PDCCH are distributed across the two RBs 1010 and the four
OFDM symbols 1020. In FIG. 10, an example of an E-PDCCH DM-RS of
two antenna ports is shown. If the E-PDCCH is transmitted with only
rank one, then the adjacent DM-RS RE pairs can be reduced to one
DM-RS RE. In one embodiment, the E-PDCCH DM-RS are transmitted with
an E-PDCCH region-specific transmission format (i.e., precoding,
rank and transmit power) which is aligned with the E-PDCCH
transmission format in the corresponding E-PDCCH region. The
E-PDCCH DM-RS and the PDSCH DM-RS 1040 are used separately for
demodulation of the E-PDCCH and the PDSCH, respectively. Therefore,
the E-PDCCH DM-RS and the PDSCH DM-RS 1040 can be transmitted with
different transmission modes.
[0101] From FIG. 10, it can be seen that by dividing OFDM symbols
within an RB between the E-PDCCH and the PDSCH and by sharing the
available DM-RS RE locations within the RB, the available DM-RS REs
are split between the E-PDCCH region and the PDSCH region.
Depending on the boundary between the E-PDCCH region and the PDSCH
region, the channel estimation performance may be affected if the
REs belonging to the E-PDCCH or PDSCH are too far from the
corresponding DM-RS REs. Therefore, it may be better to use all the
OFDM symbols within an RB for the E-PDCCH. This may also simplify
the signaling of the E-PDCCH resources. That is, the partition
between the E-PDCCH region and the PDSCH region might be
RB-based.
[0102] Alternatively, the E-PDCCH region and the PDSCH region
within an RB might not share the same set of available DM-RS RE
locations. The E-PDCCH DM-RS might be transmitted on a predefined
set of REs within an RB that does not coincide with the Rel-8 CRS,
the legacy control region, and the PDSCH DM-RS. In one embodiment,
the E-PDCCH DM-RS can be transmitted on OFDM symbols that are not
defined for the E-PDCCH region within an RB. Similarly, the PDSCH
DM-RS can be transmitted on a predefined set of REs within an RB
that does not coincide with the Rel-8 CRS, the legacy control
region, and the E-PDCCH DM-RS. In one embodiment, the PDSCH DM-RS
can be transmitted on OFDM symbols that are not defined for the
PDSCH region within an RB.
[0103] In an embodiment, multiple E-PDCCH regions are defined
within a subframe, and each E-PDCCH region has a region-specific
DM-RS with its own transmission format. Each E-PDCCH region may be
associated with a group of LTE-A UEs with a specific preferred
transmission format. An E-PDCCH region and its associated group of
UEs may be updated from time to time. As described with regard to
the third concept under the second solution, if a new DCI is
defined in the legacy PDCCH region to indicate the configuration
information of a particular E-PDCCH region, the new DCI might be
scrambled by the E-PDCCH group RNTI assigned to the group of UEs
associated with the particular E-PDCCH region. The eNB can signal
an LTE-A UE with the E-PDCCH region(s) and the corresponding
configuration information assigned to the UE. In an embodiment, an
LTE-A UE can also be signaled with the resource location of other
E-PDCCH region(s) in the subframe in order for the LTE-A UE to know
the boundary of the PDSCH region in different RBs within the
subframe.
[0104] It is typically preferable that the E-PDCCH region not be
punctured by the transmission of CSI-RS REs. In one embodiment, the
CSI-RS is not transmitted on RBs where the E-PDCCH region is
defined. In another embodiment, the CSI-RS is not transmitted on
the OFDM symbols within the RBs where the E-PDCCH region is
defined. In this case, in one RB which is allocated to the E-PDCCH,
only part of the CSI-RS may be transmitted. This information may be
signaled to the UE in order to ensure an accurate CSI-RS based
measurement by the UE. However, to maintain the accuracy of the CSI
measurement, the E-PDCCH region may also include the CSI-RS
transmission. In an embodiment, the allocation of the REs for the
E-PDCCH within the E-PDCCH region excludes the REs for the CSI-RS,
and the rate matching of the E-PDCCH takes that exclusion into
account.
[0105] In an embodiment of the second concept under the third
solution, uplink HARQ ACK/NACK timing for the PDSCH assigned by the
E-PDCCH is provided. As described with regard to the first concept
under the third solution, it may be preferable to use RB-based
partitioning between the E-PDCCH region and the PDSCH region. In
that case, E-PDCCH decoding at the UE may start only after the last
OFDM symbol of a subframe. If the same uplink HARQ ACK/NACK timing
as defined in Rel-8 is used (that is, the uplink HARQ ACK/NACK is
transmitted on the fourth subframe after the subframe where the
PDSCH is received), there may not be sufficient time for the UE to
decode the E-PDCCH and the corresponding PDSCH. In an embodiment,
one more subframe is added to the uplink HARQ ACK/NACK transmission
time for the PDSCH that is assigned by the scheduling grant sent on
the E-PDCCH. A UE can be configured to receive the scheduling
assignment on either the legacy PDCCH or the E-PDCCH. When a UE is
configured to receive the PDCCH, the regular uplink HARQ ACK/NACK
timing of four subframes can be used. When a UE is configured to
receive the E-PDCCH, an additional subframe can be added. That is,
five subframes of uplink HARQ ACK/NACK timing are used.
[0106] In another embodiment, a UE can be configured to receive the
scheduling assignment on either the legacy PDCCH or the E-PDCCH.
When a UE receives the PDSCH scheduling grant on the PDCCH, the
regular uplink HARQ ACK/NACK timing of four subframes can be used
to send the ACK/NACK for this PDSCH. When a UE receives the PDSCH
scheduling grant on the E-PDCCH, the HARQ ACK/NACK timing of five
subframes can be used to send the ACK/NACK for this PDSCH.
[0107] In an embodiment, the downlink HARQ timing can remain at
eight milliseconds (or eight subframes) round trip time (RTT) as in
Rel-8 if the eNB can process the uplink HARQ ACK/NACK on the fourth
or fifth subframe for the cases of PDCCH and E-PDCCH, respectively,
and schedule the retransmission in time on the eighth subframe. In
another embodiment, a downlink HARQ timing of 10 milliseconds (or
10 subframes) RTT is used when a UE is configured to receive the
E-PDCCH. In another embodiment, a downlink HARQ timing of 16
milliseconds (or 16 subframes) RTT is used when a UE is configured
to receive the E-PDCCH.
[0108] In an embodiment of the third concept under the third
solution, the E-PDCCH can be configured with different transmission
modes. For example, transmit diversity or rank one
precoding/beamforming modes may be used to improve the coverage of
the E-PDCCH. In an embodiment, in the case of rank one
precoding/beamforming, the Rel-8 mode 7 UE-RS pattern can be used
for the E-PDCCH DM-RS within the OFDM symbols defined for the
E-PDCCH region in an RB. In the case of transmit diversity, the
E-PDCCH DM-RS can reuse the same pattern as the LTE-A PDSCH DM-RS
as described with regard to the first concept under the third
solution.
[0109] The transmission mode can be configured per E-PDCCH region,
and the same transmission mode can be applied to all the E-PDCCH
transmissions within the E-PDCCH region. In another embodiment, all
the E-PDCCH regions have the same transmission mode configured. The
transmission mode associated with an E-PDCCH region can be
semi-statically configured and signaled via dedicated, multicast or
broadcast RRC signaling. Alternatively, the transmission mode can
be dynamically signaled using the new DCI described with regard to
the third concept under the second solution. In yet another
embodiment, the transmission mode of an E-PDCCH region can be
signaled by an E-PDCCH transmitted on a reference or primary
E-PDCCH region.
[0110] In an embodiment of the fourth concept under the third
solution, cross carrier indication of the E-PDCCH region is used.
As described with regard to the second solution, the E-PDCCH region
can be used to avoid strong interference on the legacy control
region. Therefore, instead of using the new DCI transmitted on the
legacy control region on the same CC to indicate the E-PDCCH region
configuration information, the new DCI can be transmitted on the
legacy control region of another CC. This other CC does not
experience severe interference on the legacy control region. A
carrier indication field (CIF) is added to the new DCI to indicate
the carrier where the E-PDCCH region is transmitted.
[0111] In another embodiment, cross carrier indication and
scheduling can also be applied to the E-PDCCH where a transmission
of the E-PDCCH on one CC indicates an E-PDCCH region on another CC
and/or a PDSCH assignment on another CC. In this case, the UE
attached to the low power node only monitors the E-PDCCH region it
is assigned in order to save the UE's power. When cross indication
is used, the resource location of the E-PDCCH region(s) of another
carrier is signaled to the UE as well so that the UE can correctly
decode the PDSCH assignment on another carrier.
[0112] An embodiment of the fifth concept under the third solution
involves transmission of the E-PDCCH on the blanked/muted OFDM
symbols by a macro eNB within its legacy control region or the
PDSCH region. As described with regard to the fifth concept under
the second solution, a macro eNB may be able to limit the number of
OFDM symbols within its legacy control region or the PDSCH region.
In an embodiment, the E-PDCCH can be transmitted by the low power
node on the OFDM symbol that corresponds to this blanked OFDM
symbol of the macro eNB. Since this OFDM symbol would experience
improved SINR, E-PDCCH performance can be improved without the
additional reservation of resource blocks in the PDSCH region of
the macro eNB for inter-cell interference coordination purposes.
However, since one OFDM symbol is reserved for the E-PDCCH,
spectral efficiency would be degraded if a small number of DCIs are
transmitted with the E-PDCCH. In one embodiment, multiple OFDM
symbols are blanked/muted by the macro eNB, which allows multiple
OFDM symbols to be used for the E-PDCCH at the low power node. It
is also possible that only some of the subcarriers of the OFDM
symbols are blanked/muted by the macro eNB to allow E-PDCCH
transmission at the low power node.
[0113] In one embodiment, the third solution and fourth solution
described above on E-PDCCH and their associated concepts can also
be applied to the macro cell, where the macro cell can transmit
E-PDCCH in the cases of heterogeneous network deployment and
homogeneous network deployment.
[0114] In an embodiment, a fourth solution addresses the case where
a UE is attached to a macro cell and experiences PDCCH interference
from a femto cell associated with a CSG. When a low power node is a
CSG cell, a UE that does not have a subscription to the CSG might
attach to a macro cell. If the UE is close to the low power node,
the UE might experience strong interference from the low power
node. In order to reduce or avoid the interference from the low
power node on the control channels (PDCCH, PCFICH, PHICH)
transmitted by the macro cell, in one embodiment, the low power
node blanks the control channel region on one or more of its CCs.
UEs served by the CSG cell might receive scheduling assignments on
the E-PDCCH on those CCs. Alternatively, UEs served by the CSG cell
might receive cross carrier scheduling assignments from the PDCCH
transmitted on those CCs whose control channel region is not
blanked. In the latter case, the E-PDCCH may not be needed, and
only the cross-carrier scheduling might be relied on. On those CCs
where the CSG cell blanks the control channel region, legacy Rel-8
UEs cannot be supported. In another embodiment, the legacy control
region of the low power node is not blanked/muted. An LTE-A UE that
is served by the macro cell and experiences severe interference
from the low power node on a CC will receive PDSCH and PUSCH grants
on the E-PDCCH transmitted by the macro cell on the CC.
[0115] In an embodiment, a fifth solution that addresses the PDCCH
interference issues in heterogeneous networks and the drawbacks of
the existing solutions for those issues involves applying control
region blanking or transmit power reduction to certain chosen
subframes. This solution addresses the PDCCH interference problems
described above while limiting the impact of PDCCH blanking/muting
or reducing the PDCCH transmit power to certain configured
subframes, such as MBSFN subframes or LTE-A subframes.
[0116] In an embodiment under the fifth solution, instead of
blanking the control channel region on the entire CC, the macro eNB
or the low power node (for the case of a CSG cell) applies blanking
only on the control region of certain chosen subframes. In this
way, the impact to the PDSCH and PUSCH scheduling assignment from
the macro eNB or low power node is limited to only those chosen
subframes. On the chosen subframes where the control channel region
is blanked from the dominant interferer (i.e., the macro eNB or the
low power node), the interfered low power node or macro eNB can
schedule the UEs that are farther away from itself and closer to
the dominant interferer.
[0117] It may be preferable that the chosen subframes, where the
macro eNB or low power node blanks the control channel region, not
be used to transmit system information or paging by the macro eNB
or low power node, respectively. The chosen subframes could be in a
fixed pattern within a certain time duration, and the fixed pattern
could be activated or deactivated with an "activate" command or a
"deactivate" command or could be dynamic on the subframe level. In
the case of a fixed pattern, an example could be that the chosen
subframe starts from subframe index 3 of a particular radio frame,
repeats every 10 subframes, and then stops after 303 subframes.
This is beneficial in supporting the semi-persistent scheduling
(SPS) service. The activation and deactivation could be done over
the PDCCH using the virtual CRC concept, where additional fields in
a DCI are set to known specified values in order to increase the
detection probability of a false positive PDCCH detection.
[0118] Rel-8 UEs may be able to be supported on the CC where
control channel region blanking is applied on certain chosen
subframes if those subframes are configured by the eNB via
broadcast signaling as MBSFN subframes and if the CRS is
transmitted on the first two symbols of each MBSFN subframe.
[0119] In another embodiment under the fifth solution, instead of
reducing the control channel transmit power on the entire CC as
proposed in the first solution, the macro eNB reduces the control
channel transmit power on certain chosen subframes. In this way,
the macro eNB can still serve some of its UEs which are close to
the macro eNB on those subframes. In one embodiment, those chosen
subframes can be MBSFN subframes so that the control channel
coverage to Rel-8 UEs is not affected on regular subframes. The
transmit power of the CRS in the first two symbols of these MBSFN
subframes is not reduced so as not to impact the channel estimation
performance of the Rel-8 UEs. On those chosen subframes, the low
power node can schedule its UEs which are farther away from the low
power node and closer to the macro eNB. It may be preferable that
those chosen subframes not be used to transmit system information
or paging by the macro eNB. The chosen subframes could be in a
fixed pattern within a certain time duration and the fixed pattern
could be activated or deactivated with an "activate" command or a
"deactivate" command or could be dynamic on the subframe level. In
the case of a fixed pattern, an example could be that the chosen
subframe starts from subframe index 3 of a particular radio frame,
repeats every 10 subframes, and then stops after 303 subframes.
[0120] In an embodiment, a sixth solution that addresses the PDCCH
interference issues in heterogeneous networks and the drawbacks of
the existing solutions for those issues involves a low power node
transmitting on MBSFN subframes of a macro cell. This solution
addresses the PDCCH interference problems described above while
leveraging the existing MBSFN subframes feature to avoid PDCCH
blanking/muting on the macro cell.
[0121] As in Rel-8, the control region for MBSFN subframes is
limited to a maximum of two OFDM symbols. If the macro cell is
already using MBSFN subframes for some purpose (e.g., using MBSFN
subframes to support LTE-A UEs or to send MBMS (Multimedia
Broadcast/Multicast Service) data), the macro eNB can provide the
MBSFN subframe configuration to the low power nodes within its
coverage area via X2 or S1 signaling or via a SON manager. In an
embodiment, a low power node may detect the MBSFN subframe
configuration of a macro cell by reading SIB2 of the macro cell
(plus SIB2 of any neighboring cells) to look for the
mbsfn-SubframeConfigList information element. The neighCellConfig
information element in SIB3 could also potentially be used to infer
the MBSFN subframe configurations of intra-frequency cells
neighboring the macro cell.
[0122] Based on the information obtained in this way, the low power
cell knows in which subframes the macro cell will be transmitting a
PDCCH that is guaranteed to be shorter than three OFDM symbols. If
there are sufficient MBSFN subframes and the total traffic load on
the low power cell is not excessive, the low power cell can try to
direct most or all of its transmissions into these (macro cell)
MBSFN subframes and can use a PDCCH length of three OFDM symbols
(where at least the third OFDM symbol will not overlap with the
macro cell's PDCCH). In another embodiment, the low power cell
directs its transmissions to UEs that are more severely interfered
with by the macro cell during these (macro cell) MBSFN subframes in
order to improve the probability of success in PDCCH decoding for
those UEs. The macro cell may or may not reduce its control channel
transmission power during these MBSFN subframes. In an embodiment,
the macro cell blanks its transmission (PDSCH and/or Physical
Multicast Channel (PMCH)) on the third OFDM symbol of some or all
of the MBSFN subframes to avoid interference to the low power cell.
The impact of blanking to the Rel-8 UEs scheduled on those MBSFN
subframes can be mitigated by choosing a lower modulation and
coding scheme for the Rel-8 UEs.
[0123] In some embodiments, the macro eNB only transmits the PMCH
for MBMS data on certain MBSFN subframes. This can occur when a
continuous set of the MBSFN subframes are allocated and the
necessity to transmit the PDCCH and PHICH from the macro eNB is
avoided for some MBSFN subframes. In those MBSFN subframes, there
is no PDCCH transmitted by the macro eNB, and therefore the
interference to the control region of low power nodes is greatly
reduced. The macro eNB can inform a low power node of such MBSFN
subframes via X2 or S1 signaling or via a SON manager. The low
power node can try to direct most or all of its transmissions into
these (macro cell) MBSFN subframes. In another embodiment, the low
power node directs its transmissions to UEs that are more severely
interfered with by the macro cell during these (macro cell) MBSFN
subframes where only the PMCH is transmitted.
[0124] This approach may not work as well for ongoing uplink grants
(DCI 0) for uplink HARQ retransmissions and PHICH (Physical HARQ
Indicator Channel) transmission, due to the uplink HARQ synchronous
period of 8 ms (which does not match the MBSFN periodicity of 10 or
40 ms). Also, this approach is dependent upon the macro cell
actually using MBSFN subframes and using a sufficient density of
MBSFN subframes that the low power cell can take advantage of
them.
[0125] The low power cell may not necessarily restrict itself to
only using the macro cell's MBSFN subframes. However, if traffic
loading permits, the low power cell could try to take maximum
advantage of the macro cell's MBSFN subframes where possible and
avoid the macro cell's non-MBSFN subframes where possible, at least
for UEs that experience strong interference from the macro
cell.
[0126] In one embodiment, the macro cell and low power cell radio
frame is shifted by some number of subframes such that the macro
cell can configure MBSFN subframes on those subframes where the low
power cell transmits the SIBs. In another embodiment, the macro eNB
configures MBSFN subframes on the corresponding subframes in which
the low power cell transmits SIBs. Due to the relatively large
periodicity of the SIB transmissions from the low power node, the
macro eNB does not need to configure many MBSFN subframes for this
purpose, which may end up wasting the spectrum resource. This can
be coordinated between macro eNB and low power nodes via X2 or S1
interface signaling or via the SON manager. The S1/X2 signaling
between macro eNB and low power nodes contains information related
to when the macro eNB will configure MBSFN subframes and when the
low power nodes will transmit SIBs.
[0127] FIG. 11 illustrates an embodiment of a method 1100 for
managing control channel interference. At block 1110, an access
node transmits a first control channel at a lower nominal transmit
power on a first component carrier than the nominal transmit power
of a second control channel on a second component carrier. At block
1120, the access node transmits a data channel at the same nominal
transmit power on the first and second component carriers.
[0128] FIG. 12 illustrates an embodiment of an alternative method
1200 for managing control channel interference. At block 1210, a
first access node located within the coverage area of a second
access node transmits an E-PDCCH. At block 1220, the first access
node blanks a legacy control channel. The second access node
transmits the legacy control channel with the same nominal power as
a downlink data channel. The E-PDCCH is defined within a downlink
data channel region and transmitted by the first access node to
carry downlink data channel and uplink data channel scheduling
assignment information to UEs attached to the first access
node.
[0129] FIG. 13 illustrates an embodiment of an alternative method
1300 for managing control channel interference. At block 1310, a
first access node transmits an E-PDCCH. At block 1320, a DM-RS for
the E-PDCCH supports channel estimation of the E-PDCCH.
[0130] FIG. 14 illustrates an embodiment of an alternative method
1400 for managing control channel interference. At block 1410, a
first access node performs at least one of blanking and
transmitting at lower than nominal transmit power on at least a
portion of a control channel. At block 1420, the first access node
applies at least one of blanking and transmitting at lower than
nominal transmit power only on the control region of chosen
subframes, wherein the second access node transmits the control
region of chosen subframes at nominal transmit power.
[0131] The UEs, access nodes, and other components described above
might include a processing component that is capable of executing
instructions related to the actions described above. FIG. 15
illustrates an example of a system 1800 that includes a processing
component 1810 suitable for implementing one or more embodiments
disclosed herein. In addition to the processor 1810 (which may be
referred to as a central processor unit or CPU), the system 1800
might include network connectivity devices 1820, random access
memory (RAM) 1830, read only memory (ROM) 1840, secondary storage
1850, and input/output (I/O) devices 1860. These components might
communicate with one another via a bus 1870. In some cases, some of
these components may not be present or may be combined in various
combinations with one another or with other components not shown.
These components might be located in a single physical entity or in
more than one physical entity. Any actions described herein as
being taken by the processor 1810 might be taken by the processor
1810 alone or by the processor 1810 in conjunction with one or more
components shown or not shown in the drawing, such as a digital
signal processor (DSP) 1880. Although the DSP 1880 is shown as a
separate component, the DSP 1880 might be incorporated into the
processor 1810.
[0132] The processor 1810 executes instructions, codes, computer
programs, or scripts that it might access from the network
connectivity devices 1820, RAM 1830, ROM 1840, or secondary storage
1850 (which might include various disk-based systems such as hard
disk, floppy disk, or optical disk). While only one CPU 1810 is
shown, multiple processors may be present. Thus, while instructions
may be discussed as being executed by a processor, the instructions
may be executed simultaneously, serially, or otherwise by one or
multiple processors. The processor 1810 may be implemented as one
or more CPU chips.
[0133] The network connectivity devices 1820 may take the form of
modems, modem banks, Ethernet devices, universal serial bus (USB)
interface devices, serial interfaces, token ring devices, fiber
distributed data interface (FDDI) devices, wireless local area
network (WLAN) devices, radio transceiver devices such as code
division multiple access (CDMA) devices, global system for mobile
communications (GSM) radio transceiver devices, worldwide
interoperability for microwave access (WiMAX) devices, digital
subscriber line (xDSL) devices, data over cable service interface
specification (DOCSIS) modems, and/or other well-known devices for
connecting to networks. These network connectivity devices 1820 may
enable the processor 1810 to communicate with the Internet or one
or more telecommunications networks or other networks from which
the processor 1810 might receive information or to which the
processor 1810 might output information.
[0134] The network connectivity devices 1820 might also include one
or more transceiver components 1825 capable of transmitting and/or
receiving data wirelessly in the form of electromagnetic waves,
such as radio frequency signals or microwave frequency signals.
Alternatively, the data may propagate in or on the surface of
electrical conductors, in coaxial cables, in waveguides, in optical
media such as optical fiber, or in other media. The transceiver
component 1825 might include separate receiving and transmitting
units or a single transceiver. Information transmitted or received
by the transceiver component 1825 may include data that has been
processed by the processor 1810 or instructions that are to be
executed by processor 1810. Such information may be received from
and outputted to a network in the form, for example, of a computer
data baseband signal or signal embodied in a carrier wave. The data
may be ordered according to different sequences as may be desirable
for either processing or generating the data or transmitting or
receiving the data. The baseband signal, the signal embedded in the
carrier wave, or other types of signals currently used or hereafter
developed may be referred to as the transmission medium and may be
generated according to several methods well known to one skilled in
the art.
[0135] The RAM 1830 might be used to store volatile data and
perhaps to store instructions that are executed by the processor
1810. The ROM 1840 is a non-volatile memory device that typically
has a smaller memory capacity than the memory capacity of the
secondary storage 1850. ROM 1840 might be used to store
instructions and perhaps data that are read during execution of the
instructions. Access to both RAM 1830 and ROM 1840 is typically
faster than to secondary storage 1850. The secondary storage 1850
is typically comprised of one or more disk drives or tape drives
and might be used for non-volatile storage of data or as an
over-flow data storage device if RAM 1830 is not large enough to
hold all working data. Secondary storage 1850 may be used to store
programs that are loaded into RAM 1830 when such programs are
selected for execution.
[0136] The I/O devices 1860 may include liquid crystal displays
(LCDs), touch screen displays, keyboards, keypads, switches, dials,
mice, track balls, voice recognizers, card readers, paper tape
readers, printers, video monitors, or other well-known input/output
devices. Also, the transceiver 1825 might be considered to be a
component of the I/O devices 1860 instead of or in addition to
being a component of the network connectivity devices 1820.
[0137] The following are incorporated herein by reference for all
purposes: 3GPP TS 36.213, 3GPP TS 36.331, and 3GPP TR 36.814.
[0138] In an embodiment, a method for managing control channel
interference is provided. The method includes a first access node
performing at least one of blanking and transmitting at lower than
nominal transmit power on at least a portion of a control channel.
The method further includes the first access node applying at least
one of blanking and transmitting at lower than nominal transmit
power only on the control region of chosen subframes, wherein a
second access node transmits the control region of the chosen
subframes at nominal transmit power.
[0139] In an alternative embodiment, another method for managing
control channel interference is provided. The method includes a
second access node providing a special subframe configuration to a
first access node via at least one of X2 signaling, S1 signaling, a
self organizing network (SON) manager, and a system information
block, wherein the first access node transmits a control channel to
at least one UE during at least one of the special subframes of the
second access node.
[0140] In an alternative embodiment, a first access node is
provided. The first access node includes a processor configured
such that the first access node performs at least one of blanking
and transmitting at lower than nominal transmit power on at least a
portion of a control channel, and applies at least one of blanking
and transmitting at lower than nominal transmit power only on the
control region of chosen subframes, wherein a second access node
transmits the control region of the chosen subframes at nominal
transmit power.
[0141] In an alternative embodiment, a second access node is
provided. The second access node includes a processor configured
such that the second access node provides a special subframe
configuration to a first access node via at least one of X2
signaling, S1 signaling, a self organizing network (SON) manager,
and a system information block, wherein the first access node
transmits a control channel to at least one UE during at least one
of the special subframes of the second access node.
[0142] While several embodiments have been provided in the present
disclosure, it should be understood that the disclosed systems and
methods may be embodied in many other specific forms without
departing from the spirit or scope of the present disclosure. The
present examples are to be considered as illustrative and not
restrictive, and the intention is not to be limited to the details
given herein. For example, the various elements or components may
be combined or integrated in another system or certain features may
be omitted, or not implemented.
[0143] Also, techniques, systems, subsystems and methods described
and illustrated in the various embodiments as discrete or separate
may be combined or integrated with other systems, modules,
techniques, or methods without departing from the scope of the
present disclosure. Other items shown or discussed as coupled or
directly coupled or communicating with each other may be indirectly
coupled or communicating through some interface, device, or
intermediate component, whether electrically, mechanically, or
otherwise. Other examples of changes, substitutions, and
alterations are ascertainable by one skilled in the art and could
be made without departing from the spirit and scope disclosed
herein.
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